Oscillations of an argon plasma jet detected optically outside the arc chamber were compared with those found in the records of the arc voltage and analyzed using the fast Fourier transform. The resulting frequency spectra show significant differences in occurence of some
oscillations inside and outside the arc chamber depending on the gas flow rate. The global oscillations generated especially at high gas flow rates show also remarkable deviations when the differences of the phase angles related to two points situated at different positions along
the flow are evaluated.